1. Field of the Invention
The present invention is directed to a pharmaceutical composition comprising F(ab′)2 antibody fragments that are preferably free from albumin and of whole antibodies and also substantially free of pyrogens, and an effective amount of a pharmaceutically acceptable carrier. It is also directed to a method for the preparation of a pharmaceutical composition comprising F(ab′)2 antibody fragments using serum or blood plasma of a mammal that has been previously immunized as a source of antibodies. The serum or blood plasma is digested with an enzyme, pepsin, followed by separation and purification until the pharmaceutical composition of F(ab′)2 fragments are free of albumin and complete antibodies, and substantially free of pyrogens.
2. Background Art
Antibodies are proteins of a globulin type known as immunoglobulins that are present in blood serum as a response of the immune system to the invasion of some foreign substance or organism, and are characterized for specifically combining with those substances that are foreign to the organism, neutralizing them and precipitating them so that they are removed from circulation. Various industrial applications have been developed with them for the diagnosis, monitoring, prevention and treatment of different ailments.
In regions where, due to climatic conditions, venomous animals abound, antibodies have been given a special use to combat venom. A large number of doses are applied when treating patients with scorpion, spider and snake stings or bites, principally. At present, a use that is gaining in importance is as a treatment for auto-immune diseases like rheumatoid arthritis, immune-dependent diabetes mellitus, AIDS, hemophilic anaemias, rheumatic fever, multiple sclerosis, thyroiditis and psoriasis, among others. In these cases, anti-cytokine antibodies are applied either directly to the patient or by treating blood that has been taken from and is subsequently re-fed to the patient, in order to remove the cytokines generated by the organism itself in response to the ailment. If such cytokines are not removed, they will cause extremely troublesome symptoms (see U.S. Pat. Nos. 5,888,511 and 4,940,670).
There are several kinds of immunoglobulins, known as IgG, IgM, IgD, IgA and IgE, of which IgGs are the most abundant in the blood circulation. IgGs correspond to a mature immune response and therefore include the vast majority of antibodies that are commercially produced. All the IgGs have the same general structure (which can be seen in FIG. 1). They are composed of four polypeptide chains, two that are heavy (H) and two light (L), which are joined together by disulfide bridges. The two heavy chains, in turn, are joined together by two other disulfide bridges known as the hinge region, approximately halfway along the chains. A little closer to the amino terminal region, each heavy chain is joined by a disulfide bridge with a light chain. Each heavy chain has three constant regions, CH1, CH2, and CH3, the last two in the carboxy terminal region (before the hinge) and the first in the amino terminal region (immediately after the hinge) and a Variable region (VH) in the amino terminal end, while each light chain has only one constant region, CL, in the carboxy terminal end and one variable region, VL, in the amino terminal end.
When the IgG is digested enzymatically, different fragments are obtained depending on the enzyme used, that is, if papain is used, three fragments are obtained, the crystallizing fragment (Fc) and two antigen-binding fragments (Fab) and, if pepsin is used, one F(ab′)2 fragment is obtained, while the crystallizing fragment is digested. The foregoing is due to the fact that papain cuts the heavy chains immediately after the hinge (towards the amino terminal region), while pepsin cuts them before the hinge (towards the carboxy terminal region). Fab and F(ab′)2 fragments conserve their capacity to specifically bind to the antigen that gave rise to them. F(ab′)2 fragments also precipitate antigens, while the Fc antibody fraction normally acts as a marker signal for macrophages as well as the activation of lymphocytes for the recognition and phagocytosis of the antigen-antibody complex.
The Fc fragment comprises the antigenic determinants of the antibody in such a way that when a patient is administered whole antibodies generated in some animal of another species, the patient generates an immune response against these antigenic determinants. This may give rise to varied adverse secondary responses that can even include anaphylactic shock.
These problems are significantly reduced when the antibodies are previously digested with papain or pepsin and only the resulting purified Fab or F(ab′)2 fragments are administered.
The use of Fab or F(ab′)2 fragments has another advantage that is known as the concept of distribution volume, which is simply the volume of the body in which a determined drug is dissolved. This volume can refer to the circulating blood alone, as is the case of IgG, or can include a larger part of body water in the case of the fragments. For this reason, as Fab and F(ab′)2 have a greater corporeal volume they can neutralize toxins lodged in various tissues, not only in the blood. They can even cross the blood/brain barrier in both directions and be used to neutralize or eliminate neurotoxins.
The use of F(ab′)2 fragments has a particular advantage over the use of Fab fragments in that they are retained far longer in the organism because they have double the molecular weight. Moreover, they conserve their capacity to precipitate the antigen in physiological conditions as well as maintaining a size that allows them access to a distribution volume that is sufficient for treatment purposes.
Due to the fact that the F(ab′)2 fragments conserve the main characteristics of the antibodies, the applications of the antibodies extend to F(ab′)2 fragments, with the additional advantage that because they lack the Fc fragment, recognition as foreign by a patient to whom they are administered is less likely. This provides greater tolerance to application of F(ab′)2 fragments and reduces the possibility of secondary reactions, which is particularly useful for prolonged treatments such as those applied in autoimmune diseases.
It has been known for many years that soluble proteins (particularly serous proteins) lose solubility as the concentration of neutral salts (such as ammonium and sodium sulfates) in the solution increases. In this way, for example, euglobulin precipitates with 13.5% sodium sulfate, pseudoglobulin with 17.4% and pseudoglobulin 2 with 21.3%. This fact has been used to partially purify antibodies from serum or plasma.
Several approaches in the production of antibodies and their fragments have been reported in the literature. For example, U.S. Pat. No. 4,849,352, to Sullivan et al., claims the production of both Fab fragments through the digestion of antibodies with papain immobilized in polyacrylamide. Sullivan et al. also claims the production of F(ab′)2 fragments through the digestion of antibodies with immobilized pepsin, obtaining Fab and Fc or F(ab′)2 fragments and subsequently purifying the fragments through immunoaffinity, passing them through a polyacrylamide sieve containing the specific antigen of the antibodies in question. Later the Fab or F(ab′)2 fragments that have specifically bound to the molecules in the sieve are recovered with some strongly ionic solution. The use of immobilized enzymes for digestion and immobilized antigens for purification could prove to be extremely expensive for the large scale commercial production of preparations of antibody fragments, which is a drawback despite the purity of the fragments obtained.
Furthermore, although an antigenic sieve may be useful for producing antibody fragments against pure substances, this method is not economically feasible to produce antibodies against venoms that are mixtures of a large number of toxins, many of which have a biological effect.
Another approach is shown in U.S. Pat. No. 5,733,742 in which Landon claims a process to produce Fab fragments using whole blood in a sterile medium, in which the whole blood is put directly into contact with the enzyme, free or immobilized, that has preferably been purified. Subsequently, the cell residues are removed by centrifugation, separating and recovering the resulting fragments that are subsequently purified preferably by immunoaffinity. Again, Landon used purified antigens which, unlike venoms, can easily be bound to supports to obtain a sieve for the purification of the Fab of interest. Landon never used or discussed the method of obtaining Fab fragments against antigens that are mixtures of many substances, as is the case of venoms. He only worked with papain and chemopapain and did not discuss the possibility of using pepsin.
An additional approach to the production of Fab fragments is shown in U.S. Pat. No. 4,814,433 in which Fredrickson describes a procedure for obtaining papain free Fab. He observes that when antibodies are digested by this enzyme some contaminants remain in the solution. These contaminants are hybrid compounds of the papain joined by disulfide bridges to some of the fragments resulting from the digestion, which the papain can subsequently continue digesting and degrading the fragments obtained. In order to solve the problem, Fredrickson used antipapain antibodies, which capture the hybrid compound of the enzyme. Subsequently, the fragments were purified by passing the solution along a column with protein A in which the Fc fragments and the hybrid compounds were retained. This problem, present in the digestion with papain, has not been reported when digestion is done with pepsin.
Some traditional methods involve the digestion of pepsin and the precipitation of the fraction of the fragments with ammonium or sodium sulfates, but a pre-separation is usually done with the antibodies by precipitation with sulfate and then digestion of the antibody fraction. However, large losses have been reported of the biological activity in the resulting fragments and a high content of intact antibodies and other contaminants.
As can be seen from the background, although there are several methods for the production of Fab antibody fragments, they are often difficult to apply as immunogens in the case of complete venoms. Furthermore, the advantage of using F(ab′)2 fragments in this case is clear since they have a greater retention time than Fab and do precipitate neutralizing toxins. Moreover, the reports of F(ab′)2 fragment production by means of digestion with pepsin have given evidence of a considerable loss of biological activity and a high content of whole antibodies and other impurities, which has discouraged the commercial production of pharmaceutical products comprising this type of fragment.